As background information, assessing lymph nodes in malignant diseases (cancers) in diagnostic radiology is a major problem and one that continues to remain unresolved. The contemporary approach is based on the principle that lymph nodes with a size of more than 10 mm harbor malignancy and that lymph nodes that are less than 10 mm are benign (inoffensive). This crude morphological classification is not always correct, however, since of course lymph nodes less than 10 mm in size can also be affected by tumor cells and since lymph nodes more than 10 mm in size can also be simply enlarged due to an inflammatory change and therefore do not harbor malignancy. Furthermore there also exist fibrotic lymph nodes which have been converted into connective tissue as a result of a previous inflammation process and consequently are likewise enlarged.
Positron emission tomography (PET) is frequently used in the prior art for diagnosing lymph nodes. After a tracer, fluorodeoxyglucose (FDG) for example, has been administered, PET visualizes the local activity of the glucose metabolism, which is significantly increased in the case of malignant diseases. A disadvantageous aspect, however, is that the activity of the glucose metabolism is also increased in the case of inflammations, which means that a malignant disease cannot be inferred on the basis of an increase in the activity of the glucose metabolism. Although the use of the tracer fluorothymidine (FLT) in PET reveals a higher specificity for malignant diseases, it likewise proves unsuccessful very frequently in the case of lymph nodes. Trials with other PET tracers, based on 11C-choline for example, have similarly failed to produce good results to date and consequently they are not employed commercially.
Moreover, using PET for diagnosing lymph nodes is problematic due to the low spatial resolution. In PET, a small lymph node is usually not clearly visible due to the partial volume effect. As a result of the partial volume effect objects are represented with too little activity concentration in the PET image if their size is in the range of the PET resolution or below.
Equally, the combined use of PET and computed tomography (CT) achieves no further advantage compared to PET on its own in relation to this problem, since CT merely allows a judgment concerning the size of the lymph node and furnishes no information on the reason for the enlargement (e.g. malignant event).
As background information, magnetic resonance tomography (MRT) is likewise only able to deliver judgments relating to the size of a lymph node, since experiments, by way of MR spectroscopy for example, to deliver judgments concerning the malignancy of a lymph node have so far yielded no satisfactory results. Also known are lymphotropic contrast agents for MRT which are taken up by inflammatory cells and selectively accumulate in the lymphatic tissue. Examples of contrast agents of the type are iron oxides, such as Sinerem (Guerbet) and perfluorocarbons (experimental contrast agents). However, the contrast agents likewise do not lead in every case to a correct diagnosis, since they merely indicate if inflammatory cells (macrophages) are present in a lymph node and give no direct pointer to whether malignant cells are contained in a lymph node. If a lymph node is fibrotic and contains neither macrophages nor tumor cells, the use of such contrast agents can lead to an incorrect diagnosis in that the lymph node is wrongly classified as cancerous.
The use of iron oxides as contrast agents results in a further disadvantage, since iron oxide leads to a negative contrast (to a local signal loss) so that the lymph nodes are no longer visible. For the assessment according to the prior art it is therefore necessary to acquire a second MR image which is generated before the contrast agent is administered (i.e. at least 24 hours previously). Since the lymph nodes can shift position within the course of 24 hours (i.e. between the generation of the two MR images), a corresponding evaluation is problematic. Other MR contrast agents (e.g. gadolinium chelates) produce only a very unspecific contrasting of the lymph nodes and therefore lead to no satisfactory results in the assessment of the malignancy.